Method of moistureproofing regenerated cellulose



Patented Dec. 5, 1939 PATENT OFFICE METHOD OF MOISTUREPROOFING REGEN- ERATED CELLULOSE Charles Forbes Silsby, White Plains, N. Y., a:- signor to The Solvay Process Company, Syracuse, N. Y., a corporation of New York No Drawing. Application August 16, 1937, Serial No. 159,438

6 Claims.

This invention relates to moistureproof materials and to a process of producing the same. More particularly, it relates to moistureproof sheets or films of regenerated cellulose and 'to a process of producing the same.

Sheets or films of regenerated cellulose are ordinarily rendered impermeable to the passage of substantial amounts of moisture by coating them with a cellulose ester lacquer containing preferably cellulose nitrate, a wax, a plasticizer, and a blending agent dissolved in a suitable solvent. After the application of the lacquer, the sheets are maintained at an elevated temperature to evaporate the solvent and produce uniform and moistureproof coatings thereon. This procedure has the disadvantage that it must be carefully controlled, especially with regard to the relative amounts of the ingredients used, in order that transparent, flexible, and non-tacky products may be obtained.

It has also been proposed to produce moistureproof sheets or films of regenerated cellulose by a process which comprises removing substantially all of the water from ordinary sheets of regenerated cellulose, impregnating the dehydrated sheets with a composition comprising a wax and plasticizer such as tricresyl phosphate, evaporating the solvent from the impregnated sheet at an elevated temperature, and coating the sheets with a lacquer or a varnish to give them a desirable feel. However, such a process entails the use, of relatively large amounts of plasticizers, such as tricresyl phosphate or dibutyl phthalate, which are expensive compounds.

It is an object of this invention to provide wrapping materials comprising sheets or films of regenerated cellulose impregnated with 8. moistureproofing composition.

It is another object of this invention to provide a process for the production of moistureproof, transparent, flexible, and non-tacky sheets or films of regenerated cellulose.

It is a further object of this invention to produce moistureproof sheets or films of regenerated cellulose by a relatively simple and inexpensive process.

Other objects of this invention will be obvious from the detailed disclosure thereof.

As is wellknown, sheets or films of regenerated cellulose are ordinarily manufactured by extruding a solution of viscose througha slit into a suitable coagulating bath, after which they are washed, desulfured, washed, bleached, again washed, and finally dried. Such sheets may also be manufactured by forming a cuprammoniumcellulose solution, extruding the solution through a slit into a suitable coagulating bath, washing the sheet so formed, and drying. In this connection it should be noted the drying step does not remove all of the water from the sheet, but merely removes surface water so that the sheet feels dry, but actually contains a substantial amount of water which serves to maintain the necessary flexibility of the sheets. To prevent the sheets from becoming brittle due to evaporation of this 10 water over a period of time, they are ordinarily subjected to a glycerinating treatmentto incorporate a non-volatile hygroscopic substance therein to render them permanently flexible.

In accordance with the present invention, sheets of regenerated cellulose are rendered moistureproof by passing them first through a bath of a liquid miscible with the substances contained in the sheets; i. e., water and/or glycerine, at a rate such that only those portions of these substances in the sheets adjacent the surfaces thereof are removed, then passing the sheets immediately into another bath comprising a solution of a moistureproofing agent in a solvent miscible with the liquid used in the first bath, thus impregnating the surface layers of the sheets or films with the moistureproofing solution, and evaporating the solvent from these surface layers so as to render the sheets moistureproof. According tothis invention sheets may be treated directly after the conventional drying treatment of their manufacture described above, but preferably the dry sheets are glycerinated to incorporate a non-volatile flexibilizing agent therein before treatment according to this invention. The initial treatment to remove liquids from the surfacelayers of the sheets may be effected by merely drawing the sheets through the treating liquid at a suitable rate. The subsequent treatment with the solution of the moistureproofing agent may also be carried out by drawing the sheets through the. solution and should be done immediately after the initial treatment to avoid substantial diffusion of the water and/or glycerine from the interior of the sheets to the surface before the incorporation of the moistureproofing agent in the surface layers. Finally, the solvent for the moistureproofing agent may be evaporated fromythe sheets in any suitable manner such as by passing the sheets through aheated chamber at a temperature equal to or abovethe boiling point of the solvent, whereby the moistureprooflng agent is deposited inthe surface layers of the sheets, rendering them moistureproof. It may be desirable to effect the evaporation of the solvent at a temperature above the melting point of the moisture-proofing agent so that any pores or interstices left in the sheet by the evaporation of the solvent will be filled up 5 by the fiowable moistureproofing agent. Sheets of regenerated cellulose treated according to this invention are rendered moistureproof and their flexibility is in nowise impaired as water and/or glyerine still remain in the center of the sheets and thus effectively maintain their flexibility. Furthermore, the transparency of the sheet is not affected as the moistureprooflng agent used according to this invention is itself transparent.

The liquid used to remove the flexibilizing liquids from the surface layers of the sheets should preferably be miscible with both water and glycerine as either or both of these liquids may be present in the sheets of regenerated cellu-' lose treated according to this invention. Furthermore, this liquid should be miscible with the moistureproofing solution with which the sheets are subsequently treated in order that the moistureprooflng agent may suitably penetrate the surface layers of the sheets. Because of its miscibility with both water and many organic liquids including glycerine, acetone is particularly useful for the purposes of this invention. However, it is to be understood this invention is not limited to the use of acetone for the above purpose but any other solventhaving the above properties is equally suitable.

The initial treatment to remove water and/or glycerine from the surface layers of the sheets may be suitably effected at room temperatures 85 but, if desired, temperatures higher or lower than room temperature may be used. The time of this initial treatment is dependent to some extent on the liquid used and also to some extent on the temperature at which the treatment is conducted and thus may vary considerably. For any given installation the time of immersion of the sheets in the liquid is regulated so that water and/or glycerine are removed from only the surface layers of the sheets; for example, to a depth of about 10% of the thickness of the sheet. That is, for a sheet having a thickness of 0.001 inch the water and glycerine may be removed, for example, from each surface of the sheet to a depth of 0.0001 inch. By this initial treatment water and/or glycerine are removed from the surface layers of the sheets but remain in the body of the sheets so that the flexibility of these sheets is not substantially diminished.

Immediately after the above described initial treatment, the sheets are-immersed in a solution of a moistureproofing agent. The moistureprooflng agent used according to this invention should be transparent and soluble in a solvent miscible with the liquid used in the initial treatment. Materials suitable for use as moistureproofing agents according to this invention are substances such as petroleum oils of the liquid paraffin type, petroleum derivatives such as petrolatum (vaseline), and soluble and flexible alkyd resins such as a succinic acid-glycerol resin. However, it is to be understood thisinvention is not limitedto the use of these substances. since any suitable moistureprooiing substance or composition may be used in the practice of this invention.

The moistureproofing solution is prepared by merely causing the moistureproofing agent to dissolve in a solvent which is miscible with the liquid used in the initial treatment. The solvent used to dissolve the moistureprooflng agent may be the same substance as the liquid used in the above described initial treatment but, of course, this is not essential. However, acetone has been found to be suitable as a solvent for the moistureproofing agent as well as suitable for use in the initial treatment, as above mentioned. In some cases it may be necessary to carry out the solution of the moistureproofing agent at elevated temperatures in order to dissolve substantial amounts of the agent in the solvent,in which case it may be desirable to maintain a pressure above atmospheric over the solution to prevent substantial losses of the solvent by evaporation.

'The concentration of the moistureproofing agent in the solution may vary widely and this invention is not intended to be limited to any particular value, as this factor is obviously dependent both on the agent itself and the solvent used.

The sheets, after being subjected to the initial treatment to remove liquids from the surface layers, are immediately immersed in the moistureproofing solution, preferably a hot solution of petrolatum (vaseline) in acetone, in order to impregnate the surface layers of the sheets therewith. The temperature at which this treatment is conducted may be room temperature, or any other suitable temperature. If it is necessary to use elevated temperatures in order to retain substantial amounts of the moistureproofing agent in solution during the impregnation step, it is advisable to maintain a pressure above atmospheric over the solution to prevent substantial losses of the solvent by evaporation during the impregnation, which might result in precipitation of the moistureproofing agent. The time of immersion may vary greatly, depending somewhat on the concentration of the moistureproofing agent in the solution and somewhat on the temperature of the solution, but it is a simple matter to regulate the time of immersion so that suflicient impregnation of the surface layers of the sheets to ultimately produce moistureproof sheets is effected.

After impregnation of the surface layers with the moistureprooflng solution, the sheets are withdrawn from the solution and the solvent evaporated therefrom, preferably at an elevated temperature; e. g., by passing the sheets through a heated chamber or in any other suitable manner. The sheets may be subjected to a vacuum, if desired, in order to accelerate evaporation of the solvent. Upon evaporation of the solvent, the moistureprooflng agent is deposited in the surface layers of the sheets, and the sheets are thereby rendered moistureproof. In some cases it may be desirable to conduct the evaporation of the solvent at a temperature above the melting point of the moistureproofing agent.

Thus it is seen the products of the invention are sheets of regenerated cellulose which are transparent, flexible, and moistureproof. Any tackiness due to the presence of moistureproofing agent on the surfaces of the sheets may easily be removed by coating the sheets with a lacquer comprising a substance such as cellulose nitrate, thus rendering the products of this invention suitable for use as wrapping materials.

It is seen from the above description that according to this invention sheets or films or regenerated cellulose are produced which are not only transparent, flexible, and non-tacky, but which have moistureproof qualities equal to those of commercial moistureproof sheets now on the market. By the practice of this invention such sheets are obtained by a relatively simple and inexpensive process. Inasmuch as the water and/or glycerine are not removed from the center of the sheets according to this invention, ad vantage may be taken of their flexibilizing properties for regenerated cellulose sheets to dispense with the necessity of using expensive plasticizers incidental to the practice of prior impregnation processes. Sheets of regenerated cellulose treated according to this invention, because of the above mentioned characteristics, are suitable for wrapping articles, such as tobacco products, cakes, and candy, which tend to be damaged or otherwise detrimentally affected by substantial gains or losses of moisture and, therefore, will find wide use for such purposes.

I claim:

1. A method of moistureproofing sheets of regenerated cellulose which comprises treating said sheets with a liquid miscible with the liquids contained in said sheets to substantially completely replace the liquids in the surface layers only of said sheets with said first-mentioned liquid, then impregnating said sheets with a solution of a moistureprooflng agent in a solvent miscible with the liquid used in the first step, and evaporating the solvent from the sheets.

2. A method of moistureproofing sheets 01' regenerated cellulose containing glycerine as a flexibilizing agent, which comprises treating said sheets with a liquid miscible with the liquids contained in said sheets to substantially completely replace the liquids in the surface layers only of said sheets with said first-mentioned liquid, then impregnating said sheets with a solution of a moistureproofing agent in a solvent miscible with the liquid used in the first step. and evaporating the solvent from the sheets.

3. A method of moistureprooflng sheets of regenerated cellulose containing glycerine as a flexibilizing agent, which comprises treating said 19 sheets with acetone to substantially completely replace liquids in the surface layers only of said sheets with acetone, then impregnating said sheets with a solution of a succinlc acid-glycerol resin in acetone, and evaporating acetone from 14 the sheets to obtain transparent, dry, and flexible products.

5. A method of moistureproofing sheets of regenerated cellulose containing glycerine as a flexibilizing agent, which comprises treating said sheets with a liquid miscible with the liquids contained in said sheets' to substantially completely replace the liquids in the surface layers only of said sheets with said first-mentioned liquid, then impregnating said sheets with a solution of petroflatum in a solvent miscible with the liquid used in the first step, and evaporating the solvent from the sheets.

6. A method oi moistureprooflng sheets of regenerated cellulose containing glycerine as a flexibilizing agent, which comprises treating said sheets with a liquid miscible with the liquids contained in said sheets to substantiallycompletely replace the liquids in the surface layers only. of

said sheets with said first-mentioned liquid, then impregnating said sheets with a solution of a succinlc acid-glycerol resin in a solvent miscible with the liquid used in the first step, and evaporating the solvent from the sheets.

CHARLES FORBES SILSBY. i i 

